The fuselage compartment of an aircraft is enclosed by an outer shell, which generally includes a metal skin attached to a metal structure comprising a number of axially spaced apart curved frame members that are rib like and a number of circumferentially spaced apart axially extending straight stiffener members. The curved frame members are normally called "frames" and the straight stiffener members are normally called "stringers". Since temperatures within the fuselage compartment must usually be controlled in order to ensure a proper environment for occupants and cargo, most fuselage shells also include some form of thermal insulation that is placed adjacent the inside surface of the fuselage shell between the frames. Insulation typically used to insulate an aircraft comprises layers of fiberglass placed between two layers of water-resistant film made of materials such as polyvinyl fluoride, polyester and the like to form what is called an "insulation blanket." Such insulation blankets are usually about one inch thick and may be formed by sewing, taping, or heat sealing the two film layers together at the edges of the insulating material to encapsulate the insulating material between the film layers. While the use of water-resistant materials as the blanket cover retards moisture ingress into the insulation, the film layers typically comprise a plurality of perforations for accommodating contraction and expansion of the air trapped within the insulation blanket during the operation of the aircraft, e.g., during its ascent and descent. Therefore, the insulation blankets are not air tight or water proof.
Condensate is formed within an aircraft when the air within the fuselage compartment is cooled to a temperature below its dew point, causing the water constituent of the air to condense. This is known to occur within the fuselage compartment of the aircraft at a location at or near the inside surface of the fuselage skin. The location where such condensation takes place depends on the temperature profile of the air as you move away from the inside surface of the skin toward the center of the aircraft. The amount of condensate formed within the aircraft depends on several variables such as the relative humidity of the air within the aircraft and the temperature outside the aircraft which determines the temperature of the aircraft skin.
Accordingly, depending on the temperature profile and the particular conditions, condensate may be formed at the inside surface of the fuselage skin or within a short distance away, including within the insulation blanket. Condensate that is formed at the inside surface of the skin of the aircraft can be absorbed by the adjacent insulation by means of the condensate passing through the perforations in the insulation blankets' film surface. Both the condensate which enters the insulation blanket and that which forms within does not freely flow from the blanket due to the small size of the perforations and the related tendency for the perforations to plug up. Thus, instead of minimizing condensate absorption by the insulation and enhancing condensate management within an aircraft, the use of water-resistant films to encapsulate the fuselage insulation has aggravated the problem, resulting in the aircraft carrying a number of bulging, water-filled insulation blankets.
Ideally, the condensate formed within the aircraft should be allowed to flow along the inside surface of the fuselage free of the insulation blanket to a common collection point within the plane, e.g., to the bilge, where it can be collected and then removed. The absorption of condensate by the insulation within the insulation blankets prevents the effective elimination of condensate from the aircraft. Condensate absorption by the insulation is undesirable because it reduces the effectiveness of the insulation to insulate the passenger compartment of the aircraft from heat loss through the cold fuselage skin. Condensate absorption by the insulation also adds unnecessary weight to the aircraft, since it remains on the aircraft instead of being removed therefrom. In a Boeing 747 aircraft, for example, the added weight due to absorbed condensate can be as much as 600 pounds, which can increase the operating cost of the aircraft by about $80,000 per year.
Ideally, air and water vapor should be allowed to circulate between the skin of the aircraft and the insulation blanket. This circulation promotes the removal of small droplets of condensation that are not large enough to drip and flow on their own, by allowing vapor drying to occur. To provide for this drying, a space must be provided between the skin of the aircraft and the insulation blanket, and air must be allowed to flow freely within this space.
In addition to the above-described problems associated with condensate (water) absorption by the insulation blankets because the blankets rest against the inside surface of the fuselage skin, the insulating materials also absorb other types of liquids that may have been spilled into the cargo compartment. Such spillage is common and again results in decreased insulating effectiveness, increased weight, and the possible formation of biological growth and the resulting release of unpleasant odors into the aircraft.
Ultimately, the absorption of condensate and other liquids by the insulation requires the frequent replacement of the insulation blankets, which increases downtime, thereby adding to the costs of operating the aircraft in addition to costs which are added as a result of carrying extra weight.
Accordingly, the need exists for a device which is capable of reducing the amount of condensate and other liquids that are absorbed into and/or retained by the insulation blankets installed in the aircraft fuselage. The device should also permit any condensate formed at or within the insulation blanket to flow freely away from the blanket to the inside surface of the fuselage skin to where it can be routed downwardly to the bilge where it can be removed from the aircraft. The device should be constructed in such a manner to accommodate its convenient use and application during the assembly of a new aircraft as well as during repairs or retrofits of old aircraft.